Duplexers are an essential part of 3G cellular communication handsets. The overall purpose of a duplexer is to allow both the transmit (TX) and receive (RX) portions of a cellular radio to share a common antenna (ANT). Typically, a duplexer is a passive device with three ports: (1) a transmit port that connects to the output of the power amplifier (PA) stage of the radio, (2) a receive port that connects to the input of the low-noise-amplifier (LNA) receive stage of the radio, and (3) an antenna port which connects directly to the handset's antenna.
At present, surface acoustic wave (SAW) technology is the most ubiquitous for cellular handset duplexer applications. However, the requirements for some communications standards (notably 3G-PP Bands 2, 3, and 8) are extremely difficult or impossible to meet with standard SAW devices. The problem with respect to these bands is the very narrow frequency separation between the transmission and receiver bands.
This separation in frequency is usually referred to as the “transition band.” The steepness of the filters in this region is proportional to their quality factor (Q). Even SAW filter having Qs in the neighborhood of 300-500 cannot meet the rigorous requirements of the aforementioned 3GPP bands. Not only do those bands require very narrow transition bands, the problem is exacerbated by the necessary addition of margins for manufacturing and temperature variations.
Thus, there is a need for an improved duplexer that can meet the requirements of the 3GPP bands.